The use of coaxial cables of either the foam or air dielectric type is widespread for antenna feeding arrangements in communication systems. Typical applications include antenna systems for terrestrial microwave systems, cellular and land mobile radio, broadcast transmitting antenna systems, earth-station antenna systems, and high-frequency communication systems. Such coaxial cables function essentially to transmit electrical signals from a generating station to some form of antenna from where the signals are radiated. Coaxial cables of the radiating kind, on the other hand, are designed to themselves functions as continuous antennas so that electrical or radio signals are transmitted directly from the cables rather than from an antenna. Such radiating or "leaky" coaxial cables serve as efficient and economical sources for transmitting radio signals where the use of conventional antennas is impractical. Radiating cable systems are particularly indispensible in two-way mobile radio, radio paging and other localized broadcasting services in applications involving extended underground installations such as railways, mines and tunnels where conventional centralized VHF and UHF communication systems are not practical.
Regardless of the particular application, a common requirement of coaxial cables is high retardancy to flame propagation. Over-heating of cables when subjected to current overloads or related system failures can initiate fires. More importantly, when electrical equipment has already been subjected to fire, the cables used therein may themselves contribute to flame propagation and also produce noxious fumes and smoke. Foam dielectric coaxial cables are particularly suited to antenna feeder systems which do not require a pressure path to the antenna and are hence often specified in applications using land mobile radio, cellular radio, or terrestial microwaves links; in such applications it is important that the cables do not in any way contribute to flame propagation in case of fire.
For quite some time coaxial cables have been afforded flame retardant properties by sheathing cables with halogen-containing materials such as polyvinyl chloride (PVC) or other flouroplastic materials. Such cables resist fire propagation even under severe heat conditions; however, upon being exposed to fire the halogen containing materials in the sheaths generate noxious smoke and form toxic and corrosive gases. Beside being a substantial safety hazard, the use of such cables leads to secondary damages resulting from degrading of the fire-retardant material.
Flame retardant cables based on halogen-free materials such as olefin-copolymers and other high oxygen index materials have subsequently been developed. Improved flame retardant and fire resistant properties are provided by such cables by the process of cross-linking the halogen-free materials. A major problem with such cables is that they are extremely expensive and generally stiff and unpliable.
A problem peculiar to radiating cables of the foam-dielectric type arises due to the very construction of such cables. In a radiating cable, slots or other apertures are provided in the outer conductor to allow a controlled portion of the transmitted RF signal to radiate, thus creating elemental radiating sources along the entire length of the cable. The outer conductor itself surrounds an assembly consisting of a foam core extruded onto an inner conductor. The entire coaxial assembly is then jacketed with a flame retardant material. With this type of construction, when the cable is subjected to high heat conditions in a fire, the foam inside the cable melts and bubbles out of the apertures in the outer conductor and can penetrate the softened external jacket so as to be exposed to the fire. Consequently, flames propagate rapidly along the cable and can lead to total destruction of the cable. As a result, most existing radiating cables are incapable of passing stringent flame tests such a the IEEE 383 test.